Tox is just the core, the part of the program which handles the messages and encryption, for instance. But it can only function in the context of a complete instant messaging program. All the fancy user interfaces you see all over the place, are the clients. The client is a separate program which utilizes the core, providing a convenient way to use the program. So when you use Tox, you are in fact using one of the clients. The most clients are graphical clients, in other words, you will make use of all kinds of graphical elements which are part of the client. You usually won’t get to see the core (unless you look into the source on GitHub). There are various clients available which each have a different name, are independent from each-other, in various programming languages.